This invention relates to circuits for starting and operating discharge lamps and more particularly to a novel auxiliary network for use in combination with a standard ballast circuit which permits the ballast circuit to reliably start and operate one or more so-called T8 fluorescent lamps.
The two-lamp, series-sequence rapid start ballast is used extensively in commercial and industrial lighting systems in the United States. One type of fluorescent lamp currently used with this ballast is the standard T12 (1.5 in. diameter), 4 ft. long, 40 W lamp (designated, for example, F40CW). Standard 40 W T12 fluorescent lamps contain about 2 Torr Ar plus a small amount of Hg. They operate nominally at 105 V (RMS) and 425 ma (RMS). There is presently available a two-lamp rapid start ballast designed to start two such lamps in series sequence and to operate them in series with a 120 V AC, 60 Hz ballast input. The electron loss rate is higher in T8 lamps than it is in T12 lamps (increased ambipolar diffusion rate). In order to maintain the discharge, the electron production rate must be increased in T8 lamps by increasing the applied electric field. Such a lamp will then operate at increased power loading so that it would not be compatible with the existing T 12 ballasts.
It would be desirable to be able to substitute the new 4 ft. long T8 lamp (1.0 in. diameter) as a direct replacement for the T12 lamp since the T8 lamps have several advantages over T12 lamps. For example, the T8 lamps have higher efficacies than standard T12 lamps and they can be manufactured at a significantly lower cost than T12 lamps because they require only two-thirds as much glass and phosphor. Additionally, shipping and warehousing costs are greatly reduced for T8 lamps since many more T8 lamps can be shipped in a given size container.
Although it is easy to remove the T12 lamps from a luminaire and replace them with T8 lamps, a direct substitution is not possible because the T8 lamps are not compatible with the conventional T12 ballast. Therefore, to design a new ballast for the T8 lamps or to rewire the T12 ballast to accommodate T8 lamps would not be practical as any economic gain from the use of the T8 lamps would be offset by the higher cost of a new or rewired ballast device. Several problems occur when the smaller diameter T8 lamps are used with the standard T12 ballast. One problem is that the T8 lamps do not start reliably using the T12 ballast since the T12 ballast design was optimized for the easier to start T12 lamps. Another problem is that the operating characteristics of the T8 lamps are significantly different from the T12 lamps. If T8 lamps are connected to a T12 ballast and started by some external means, lamp voltage and current waveforms will be produced which are asymmetic and distorted, leading to considerable lamp flicker. The different operating characteristics also cause a larger RMS current to be drawn from the T12 ballast so that the lamp current exceeds the rated ballast load current by as much as 50 percent, which will lead to early ballast failure.